In recent years, a rapid spread of compact electronic device such as cellular phones and notebook personal computers created a huge demand for nonaqueous electrolyte secondary batteries as rechargeable power sources. Lithium cobalt composite oxides represented by lithium cobaltate (LiCoO2) and also lithium nickel composite oxides represented by lithium nickelate (LiNiO2) and lithium manganese composite oxides represented by lithium manganate (LiMn2O4) have been widely used as positive electrode active materials for the nonaqueous electrolyte secondary batteries.
The problems associated with lithium cobaltate are that it includes, as a major component, cobalt which is expensive due to scarce reserves and which is a metal with unstable supply and large price fluctuations. Accordingly, lithium nickel composite oxides and lithium manganese composite oxides containing relatively inexpensive nickel and manganese as the main components have attracted attention due to a low cost thereof.
However, although lithium manganate is superior to lithium cobaltate in terms of thermal stability, the applications thereof to batteries are associated with a large number of problems since the charge-discharge capacity thereof is greatly inferior to that of other materials and the charge-discharge cycle characteristic representing the service life is extremely short. Meanwhile, lithium nickelate demonstrate a charge-discharge capacity larger than that of lithium cobaltate and, therefore, is expected to be a positive electrode active material suitable for producing inexpensive batteries having a high energy density.
However, lithium nickelate is usually manufactured by mixing and firing a lithium compound and a nickel compound such as nickel hydroxide and nickel oxyhydroxide, and the product is in the form of a powder with monodispersed primary particles, or a powder of secondary particles, which are aggregates of primary particles and have voids. The drawback of either form is that thermal stability in a charged state is inferior to that of lithium cobaltate. Thus, pure lithium nickelate has not found applications in batteries for practical use due to problems associated with thermal stability and charge-discharge cycle characteristic. This is because the stability of crystal structure thereof in a charged state is lower than that of lithium cobaltate.
This problem is typically solved by substituting part of nickel with a transition metal element such as cobalt, manganese and iron, or a dissimilar element such as aluminum, vanadium, and tin, thereby stabilizing the crystal structure in a state in which lithium is desorbed in charging and producing a lithium nickel composite oxide with good thermal stability and charge-discharge cycle characteristic for a positive electrode active material (see, for example, Patent Literature 1 and Non-Patent Literature 1).
However, where the amount of the substitution element in this method is small, thermal stability cannot be sufficiently improved, and where the amount of the substitution element is large, the capacity is decreased. Therefore, the superiority of lithium nickel composite oxide cannot be realized in batteries.
Further, where a lithium nickel composite oxide is used as is after the synthesis involving firing, battery performance cannot be sufficiently demonstrated in charging and discharging due to the effect of lithium carbonate and lithium sulfate remaining on grain boundaries or the like. For this reason, impurities have been removed by washing with water (see, for example, Patent Literature 2). Washing with water has also been considered as an effective method because where the impurities present on the surface are washed off, a true specific surface area is revealed as an indicator, and correlation with thermal stability and capacity is demonstrated (see, for example, Patent Literature 3).
However, in either case, true causes of poor battery performance and mechanisms thereof have not been sufficiently clarified and sufficient capacity and output and excellent thermal stability could not be ensured only by removing the impurities and controlling the specific surface area. The resultant problem is that battery performance cannot be fully utilized.
Meanwhile, lithium nickel composite oxides use an alkali such as lithium hydroxide, and the alkali reacts with carbon dioxide in the synthesis, thereby producing lithium carbonate (Li2CO3). The resultant problem is that this compound generates gas at a high temperature and causes the battery to expand (see, for example, Non-Patent Literature 1). Further, lithium nickel composite oxides demonstrate high sensitivity to the atmosphere, and there is a concern that lithium hydroxide (LiOH) remaining on the surface even after firing could be carbonated and lithium carbonate could be further generated till the positive electrode is completely manufactured (see, for example, Non-Patent Literature 2).
Although the improvement caused by the aforementioned washing with water has been investigated with respect to the improvement of thermal stability by removing surface impurities and controlling the specific surface area, the problem of battery expanding due to gas generation has not been addressed.
A variety of methods for evaluating gas generation by a positive electrode active material have heretofore been suggested to resolve this problem (see, for example, Patent Literature 4 to 6).
However, the problem associated with Patent Literature 4 is that only the water-soluble alkali fraction revealing lithium hydroxide on the surface is specified and a lithium carbonate fraction that causes gas generation at a high temperature is not specified. Further, the problem associated with Patent Literature 5 and Patent Literature 6 is that only the lithium carbonate fraction is specified and the lithium hydroxide fraction that can change into lithium carbonate before the production of the positive electrode is completed is not specified.
With the foregoing in view, it has been necessary to resolve the problems inherent to the related art and to develop a positive electrode active material for a nonaqueous electrolyte secondary battery that combines a high capacity with excellent thermal stability and enables a high output, while clarifying true causes of poor battery performance and mechanism thereof in a positive electrode active material constituted by a lithium nickel composite oxide.